1. Field of the Invention
The present invention relates to an image forming apparatus for forming a color image and, more particular, a projector using the same.
2. Related Background Art
A conventional image forming apparatus is exemplified by a projector shown in FIG. 1.
Light emitted from a light source 30 directly or through a reflector 5 is color-separated into red, green, and blue light components by a dichroic mirror 7 for reflecting the red light component and transmitting the green and blue light components therethrough and a dichroic mirror 8 for reflecting the blue light component and transmitting the green light component therethrough and a total reflection mirror 20. The color-separated light components pass through liquid crystal light valves 1, 2, and 3 and are synthesized through a dichroic mirror 9 for reflecting the blue light component and transmitting the red light component therethrough and a dichroic mirror 10 for reflecting the green light component and transmitting the red and blue light components therethrough. The synthesized light is projected through a projection lens 6.
A metal halide lamp having spectral characteristics in the entire visible light range, as shown in FIG. 2 is used as the light source 30.
In the conventional apparatus, a color-separating optical system is located between the light source and the liquid crystal light valves, and optical paths between the light source and the liquid crystal light valves must have predetermined lengths. Illumination light from the light source cannot be perfect telecentric light and contains convergent and divergent components. For this reason, the illumination light has a considerably large loss by divergence during propagation along the optical paths having the predetermined lengths. In addition, the color-separating optical system causes an increase in the size of the apparatus as a whole, thereby degrading portability of the apparatus.
As one method of solving these conventional problems, there may be proposed a single light valve type liquid crystal projector which has one white light source and one liquid crystal light valve having an RGB mosaic color filter. This arrangement allows formation of a projector by one liquid crystal light valve, and the color-separating optical system and the color-synthesizing optical system can be omitted, thereby realizing a very compact apparatus. However, the following problems are still posed by this apparatus.
As the color filter is generally of an absorption type, the amount of white light passing through the color filter is reduced to about 1/3. For this reason, the transmittance of the liquid crystal light valve is reduced, and hence a sufficiently bright display image cannot be obtained.